The present invention relates to a fuel cell electrode that contains a catalyst layer, such as a catalyst layer of an air electrode of a fuel cell (hereinafter, also referred to as a “fuel cell air electrode catalyst”) and to a process for producing the fuel cell electrode.
Hitherto, the cathode (air electrode) of a fuel cell generally employs a noble metal, such as platinum, as a catalyst. One alternative non-platinum catalyst is nitrogen-containing carbon (i.e., carbon alloy), which is produced through high-temperature carbonization of a raw material such as polyvinyl-cobalt-phthalocyanine, iron-phthalocyanine, or porphyrin, and has become of interest by virtue of its excellent redox characteristics. However, since a carbon alloy is a carbonaceous material containing metal or nitrogen, the structure thereof is very complex. The catalytic activity of such a carbon material is thought to be regulated by a factor such as the presence of metal (Fe, Co, etc.) or nitrogen doping. However, the active site of redox reaction has not been elucidated, and the approach to enhance the density and durability of the active site has not been established. Such structural issues remain to be resolved.
In the course of research on other non-platinum carbon catalysts, a cobalt/polypyrrole/carbon complex catalyst having high redox activity has been reported to have Co—N active sites (Nature VOL. 443, 63-66 (2006)). Some iron-on-carbon catalysts have been reported to have an Fe—N4 structure fixed in pores of carbon, which structure exhibits catalytic activity (Science VOL. 324, 71-74 (2009)). In the case of a catalyst formed of carbon nanotube (CNT) particles (hereinafter, also referred to as “carbon nanotubes”), carbon atoms having an electron density reduced by N-doping and bonded to the dopant N atom serve as reaction sites (Science VOL. 323, 760-764 (2009)). In each study, nitrogen is considered to play an important role in providing catalytic activity. Thus, in actual catalyst production processes, toxic ammonia gas, expensive nitrogen-metal complex, etc. are employed.
Conventional carbon nanotubes have drawbacks such as small specific surface area and weak interaction with catalyst microparticles. In order to overcome the drawbacks, studies on CNT surface treatment methods to produce carbon nanotubes having large specific surface area (preferably equal to or higher than that of carbon black), strong interaction with catalyst microparticles, and pores in walls thereof were conducted, and as a result U.S. Patent Publication No. 2011/0206932 discloses a method for producing such CNTs.